1. Technical Field of the Invention
The present invention relates to a structure for mounting a starter motor to an internal combustion engine in which a crankcase and a transmission case are formed integrally as a power unit case.
2. Background Art
Examples of an internal combustion engine including a power unit case for housing both an engine and transmissions according to the background art, include the motorcycle engine disclosed in Published Patent Document JP-A 2001-115933.
The power unit case of the internal combustion engine disclosed in JP-A 2001-115933, in which a crankcase and a transmission case are integrally housed, is formed so as to be separable into upper and lower halves, and has a structure with a large width in a front-to-rear direction. In the power unit case of this reference, three shafts are journaled so as to be sandwiched between horizontal parting surfaces of the power unit case halves. The three shafts include a crankshaft in a crank chamber and a main shaft and a counter shaft serving as transmission shafts in a transmission chamber, and these shafts are arranged in the order of the crankshaft, the main shaft, and the counter shaft in the front-to-rear direction.
Further, a cylinder block projects from a front side portion of the crankcase while being tilted forward, and a pair of front and rear mounting bosses are formed in the rear portion of the upper wall. A starter motor is mounted to the upper wall at the front and rear mounting bosses by fastening support arms, which are provided so as to project in a front-to-rear direction of the starter motor.
Since the crankshaft, the main shaft, and the counter shaft are arranged side by side from front to rear in the horizontal parting surface of the power unit case, the power unit case has a large front-to-rear width. Accordingly, since it is not particularly necessary to raise the upper wall of the transmission case upwardly in order to accommodate gear groups on the main shaft and the counter shaft that are arranged side by side from front to rear in the transmission chamber, the space in rear of the cylinder block and above the upper wall of the power unit case is large, and hence there is no need to take possible interference between the starter motor, mounted to the rear portion of the crankcase, and the upper wall of the transmission case into consideration.
While it is easy to mount the starter motor onto the power unit case when the power unit case has a large front-to-rear width as described above, in the case where either the main shaft or the counter shaft is moved upward, and the three described shafts are brought close to each other to reduce the front-to-rear width of the power unit case to thereby reduce the size of the internal combustion engine, the space in rear of the cylinder block and above the upper wall of the power unit case becomes narrow, the upper wall of the transmission case for accommodating groups of gears is raised upward due to the upward movement of either the main shaft and the counter shaft, and also the mounting space for the starter motor becomes restricted. It therefore becomes more problematic to securely mount the starter motor to the power unit case with high mounting rigidity in such an arrangement.